Changes in serum levels of calcium, magnesium, cAMP and monoamine oxidase (MAO) activity throughout pregnancy and during prolonged treatment for preterm labor with ritodrine were studied. cAMP levels in the third trimester of pregnancy were found to be significantly high and postpartum MAO activity was significantly low. In cases of preterm labor, serum levels of calcium, magnesium and MAO activity were significantly low during the initial stage of ritodrine administration, then recovered after more than 1 week of treatment despite the fact that given doses and serum levels of ritodrine remained constant and that the serum level of cAMP was still high. These results suggested that a control mechanism for MAO activity during pregnancy might exist due to an increase in catecholamine and that the β2-effect of ritodrine induced a lowering in serum calcium and magnesium levels. However, some homeostatic mechanism to regulate the ions might come into play later on the mechanism of down-regulation of ritodrine.